Ubuntu – need to type `./` before executing a program in the current directory

bashcommand line

While executing a C program, a.out, using the Ubuntu terminal, why do I always need to type ./ before a.out, instead of just writing a.out? Is there solution for this?

Best Answer

When you type the name of a program such as a.out the system looks for the file in your PATH. On my system, PATH is set to

/usr/lib/lightdm/lightdm:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games:/usr/local/games

Yours is probably similar. To check, enter echo $PATH in a terminal.

The system looks through these directories in the order given and if it can't find the program produces a command not found error.

Prepending the command with ./ effectively says "forget about the PATH, I want you to look only in the current directory".

Similarly you can tell the system to look in only another specific location by prepending the command with a relative or absolute path such as:

../ means in the parent directory eg ../hello look for hello in the parent directory.

./Debug/hello : "look for hello in the Debug subdirectory of my current directory."

or /bin/ls : "look for ls in the directory /bin"

By default, the current directory is not in the path because it's considered a security risk. See Why is . not in the path by default? on Superuser for why.

It's possible to add the current directory to your PATH, but for the reasons given in the linked question, I would not recommend it.

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Ubuntu – always prompt the user before executing a command in the shell

Do you want it to work without typing an extra command, e.g.

$ rm file

Or only when the user types something like

$ confirm rm file

Or only when the user tries to run certain commands, e.g.

$ rm file

but not for

$ echo "Hello"

If option 1, that can be done using the preexec hook in zsh, or the DEBUG trap in bash.

If option 2, put something like this in /etc/bash.bashrc or other shell startup file.

confirm() {
    echo -n "Do you want to run $*? [N/y] "
    read -N 1 REPLY
    echo
    if test "$REPLY" = "y" -o "$REPLY" = "Y"; then
        "$@"
    else
        echo "Cancelled by user"
    fi
}

If option 3, you could modify the confirm script above, or, some commands have an option to ask before doing something, e.g. rm -i. You could put alias rm='rm -i' in /etc/bash.bashrc.

Ubuntu – Using the Terminal in Ubuntu to change directory

Arguments in the shell are are separated using spaces. This means that whenever you run a command and try to pass a file or a directory as an argument that contains a space in it, bash assumes it is two separate arguments. There are many ways to specify this. You can use what is called an escape character to signal that the space is part of the argument. To do this, just insert a \ before the space in the file name

cd Program\ Files

or you can use quotes to signal that it is a single parameter

cd "Program Files"

However, often you will not need to type the whole thing if you use the Tab Completion feature

cd P<TAB>

If there are multiple files starting with P, add another alphabet and press tab again to complete.

Best Answer

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